Many proposals have been made heretofore designed to project an enlarged film image onto a viewing surface and dividable generally into a first category for displaying images from strip film, and a second category for displaying images from a microfiche. The latter category presents far more difficult problems because of the substantially greater magnification required and, in consequence, very substantially higher precision equipment to provide a well illuminated high resolution image. Microfiche images are mounted in side by side relation on a library size card which is typically sandwiched between a pair of plates mounted on an S-Y type fiche transport. This transport is supported in a horizontal plane for movement in both directions crosswise of the optical axis. The projection lens must be firmly supported normal to the plane of the fiche. Desirably, the focal plane of this lens should be maintained precisely spaced parallel to the plane of the fiche within a tolerance of one mil, as the fiche is moved to align any fiche with the optical axis. These requirements are substantially more stringent than for strip film readers. The lens should not change position axially or laterally of the optical axis during adjustment of the fiche transport and, desirably, suitable means should be provided for changing the magnification of the image on the screen as well as means for maintaining the lighting intensity of the projected image constant as well as means for presenting the projected image in a convenient reading position whether arranged crosswise or longitudinally of the fiche card.
Prior film readers embody some but not all of these features. The 1949 British Pat. No. 616,874 discloses a reader having means supporting a film strip on a turn table but the reader lacks various other desirable features and capabilities.
Designers have proposed film readers employing projection lenses of different powers for both microfiche and film strips. Constructions of this type are shown in U.S. Pats. to Strauss No. 2,630,739, Ikezu No. 3,588,226, Bolgar No. 3,888,576, Feifer No. 3,792,226, and Peters No. 3,369,450. A reader of this type is also being marketed by Washington Scientific Industries, Inc., Long Lake, Minnesota. These various constructions employ a variety of mechanisms for substituting one lens for another of a different power. Strauss mounts his two lenses on a rotary turret head but lacks satisfactory and reliable means for maintaining the focal plane of either lens at the requisite precise distance from film supported in an X-Y film transport. In fact, such a transport is not even suggested by this patentee. Peters U.S. Pat. No. 3,369,450 had but a single stationary projection lens the power of which he endeavors to change by interposing an additional lens component. Ikezu U.S. Pat. No. 3,588,226 discloses a pair of projection lenses of different powers so designed that the focal plane of each is located at the same axial distance from the film image but he fails to show any means for supporting the two lens units and for substituting one lens for the other relative to an X-Y fiche transport. Feifer U.S. Pat. No. 3,792,923 proposes a reader having a pair of different power lenses rigidly supported on a bracket arm movable about the same vertical axis and shiftable to bring one into operating position relative to one fiche transport and the other into proper operating position relative to a second fiche transport. Neither lens can be used selectively with the same fiche transport. Bolgar U.S. Pat. No. 3,888,576 employs a pair of immovably supported lenses of different powers in combination with a linkage mechanism for adjusting a reflecting mirror system to modify the size of the displayed image.
Various expedients have also been proposed heretofore to maintain the projecting lens of a microfiche reader precisely spaced from and accurately normal to an X-Y fiche transport mechanism. Spring biasing means for maintaining a projection lens against the transport are proposed by Peters U.S. Pat. No. 3,634,005; Schutrum et al. U.S. Pat. No. 3,717,400; Peters U.S. Pat. No. 3,807,845 and Akajama U.S. Pat. No. 3,565,513. Two other patentees namely, Brownscombe U.S. Pat. No. 3,815,975 and Graef U.S. Pat. No. 3,860,333, propose accomplishing this same objective in reliance on gravity to hold the lens directly against the cover of an X-Y transport. In each instance only a single projecting lens is provided in both the gravity-actuated and the spring biased constructions. The Washington Scientific reader also utilizes gravity to support a pair of lenses of different powers on a fiche transport. However, the mechanism employed for this purpose utilizes a slider for the two lenses in combination with a T-shaped slot. The vertical T-stem of this slot is located in the plane of the optical axis and requires that the lens in use be manually lifted to the top of the T-stem portion of the slot following which the slider is shifted horizontally along the T-head portion of the slot to bring the second lens into alignment with the optical axis at which time the second lens must be manually lowered onto the film transport. This construction requires skillful manipulation to avoid risk of serious injury to the two lens components as well as to the glass plates of the fiche transport, and equally important, that the operator must manipulate the lens in a particular manner else it is impossible to substitute one for the other.